The present invention concerns laser threats identifying systems of aircrafts, and specifically a test arrangement for a system for identifying light beams impinging on an aircraft.
In avionics, to detect or track an aircraft, targeting light beams are used, generally emitted by collimated laser sources, adapted to illuminate a target and possibly to provide an aid to interfere with it.
For example, a “range finder” laser beam having the characteristics of low repetition frequency (a few kHz) and a very narrow pulse amplitude (of the order of a few ns) is used to measure the distance of a targeted object; a “designator” laser beam having the characteristics of a continuous wave with a high repetition over-modulation frequency (up to 500 kHz) and high power is used by weapon systems for targeting an object (identification and tracking); a laser beam having the characteristics of medium repetition frequency (tens of kHz) is used to illuminate a target and to direct a missile or similar laser guided weapon (“beam rider”) on it, adapted to dynamically track a predetermined targeting signal on its trajectory.
For this reason the aircrafts are equipped with on board identification systems, adapted to detect possible impinging light signals and to identify the threat posed by them, as well as the direction they come from, analysing the characteristics of the light signal and comparing them with a library of known signals, which is constantly updated.
A typical identification system is schematically shown in FIG. 1. It comprises at least one and preferably two detecting devices 10 exposed on the fairing of the aircraft 12, for example mounted symmetrically on opposite sides of the fuselage and adapted to cover all the observation directions in space. Each detector includes a faceted optical head 14 comprising a plurality of optical windows (mirrors) 16 adapted to collect one or more targeting light beams B coming from the sectors of space towards which they are oriented.
The optical signals conveyed by the collected light beams, referring to each controlled sector of space, are forwarded, through a corresponding plurality of optical channels 20, (light guides or plastic fibres) to a processing and signalling unit (Laser Warning Receiver, LWR) 30. This is provided with integrated optical sensors and adapted to analyse the characteristics of the intercepted optical signal (power and waveform of the modulating signal) and compare them with a library of known signals, keeping track of the contributions that a single incident light beam can bring simultaneously on many mirrors.
The correct detection of a threat posed by a laser beam which tracks the aircraft allows suitable countermeasures to be taken, be they a diversion flight manoeuvre, an attempt to destroy the apparatus posing the threat or the actuation of the systems for saving the pilot or the crew of the aircraft.
The system thus conceived must be tested in view of its first installation on board of an aircraft and the test arrangement can also be used with the purpose of validating the libraries the system is provided with (this is a conventional operational support function, since the libraries can vary from one mission to the next, according to the operational scenario), verifying the correct application of the priorities for identifying threats and monitoring the response speed.
The testing and the validation are based upon the generation and representation of virtual threat scenarios, adapted to replicate possible actual known threats as faithfully as possible, and the consequent verification of the response provided by the identifying system.
Currently, used test arrangements include a plurality of high power light beam generators (laser generators in free-space), arranged in different space sectors around a detection device to be illuminated, and controlled to generate optical signals at the desired wavelengths (infrared (NIR), visible) and having known form characteristics, indicative of a possible threat.
Disadvantageously, this arrangement has critical points in terms of safety and reliability. Indeed, laser generators must be shielded from one another and with respect to the environment outside the testing site. The light beams must also be correctly collimated in the air, through lenses or beam expanders, so as to replicate the propagation conditions with a widening in the atmosphere of a typical actual beam which can hit the detection device, possibly on many adjacent mirrors, and this is an operation that requires complex mechanical aligning of the laser generators.